The present invention relates to sheet resistance meters which measure the electrical resistance of a metal or alloy thin film formed by sputtering, vapor deposition, or another thin film formation technique without contacting the thin film, and also to methods of manufacturing electronic components using such meters.
A four probe technique is one of conventionally known techniques to measure the electrical resistance of a metal or alloy thin film formed by sputtering, vapor deposition, or another thin film formation technique.
The four probe technique is based on principles explained below in reference to FIG. 23. Four acicular electrodes, which constitute testing probes 52, 53, 54, and 55, are positioned on the surface of a metal film 51 formed on a substrate 50 so that their ends directly contact the surface. Here, the testing probes 52, 53, 54, and 55 are arranged in a straight line and separated from each other by some distance. A potential difference V is measured which develops between the inner testing probes 53 and 54 when an electric current I is passed through the outer testing probes 52 and 55. The resistance R (=V/I) of the metal film 51 is calculated from the measurement. Then, the resistivity xcfx81 is calculated as the resistance R multiplied by the thickness t of the metal film 51 and further by a correction factor F that is a dimensionless value determined from the shape and dimensions of the metal film 51 and the positions of the testing probes 52, 53, 54, 55.
Shortcomings, however, arise from the mechanism of the four probe technique whereby the acicular testing probes 52, 53, 54, and 55 are pressed against the metal film 51 to make direct contact with it: the metal film 51 may be damaged, which leads to production of dust particles. Also, the testing probes 52, 53, 54, and 55 per se are prone to wear due to abrasion and have to be replaced regularly.
Another problem develops with the four probe technique in the presence of vibration or shake, which obstructs the essential direct contact of the testing probes 52, 53, 54, and 55 with the metal film 51 and makes it impossible to perform measurement. A further problem with the four probe technique is related to nothing but the size of a device to execute the method. The device grows too large for various reasons, such as the inclusion of a dedicated clamp stage for measuring, to be readily accommodated in a limited installation space, especially, along with other devices in existing manufacturing lines.
To address these shortcomings, non-contact measurement is available as an alternative to the four probe technique whereby testing probes are brought into direct contact with the target object to measure the resistivity of the semiconductor material.
The technique is known as the double-sided eddy current scheme, which will be detailed here. A metal thin film is formed on a glass substrate, wafer, or other substrate for semiconductor, and the substrate is placed in a magnetic field developed by a coil to which a high frequency power is supplied. Thus, eddy currents are induced in the metal thin film due to electromagnetic effects of the magnetic field. The induced eddy currents will dissipate as Joule heat. The consumption of high frequency electric power by the metal thin film formed on the substrate has a positive correlation with the conductivity of the metal thin film. This fact provides the basis of the double-sided eddy current scheme to calculate the conductivity (the reciprocal of resistivity) of the metal thin film without contacting the thin film.
The double-sided eddy current scheme is unique over the four probe technique in that the resistivity of the metal thin film is can be calculated and evaluated without direct contact. Therefore, with the double-sided eddy current scheme, it is ensured that the metal thin film on the substrate is not damaged by direct contact, pollutants, or exertion of force in the finishing process of ICs, liquid crystal panels, and other semiconductor products.
Now, the double-sided eddy current scheme will be described by way of an example. First, as shown in FIG. 24, a high frequency electric power is supplied to a coil 62b wound around a C-shaped ferrite core 62. The ferrite core 62 has two end parts 62a which are positioned opposite to each other and separated by a 1- to 4-mm gap 61.
When a wafer 63 is inserted in the gap 61, eddy currents are induced in the metal thin film on the wafer 63 due to the high frequencies. Since the induced eddy currents dissipate as Joule heat, the supplied high frequency electric power is partly consumed by the metal thin film on the wafer 63. The consumption has a positive correlation with the conductivity of the metal thin film on the wafer 63. In the double-sided eddy current scheme, the resistivity of the metal thin film on the wafer 63 is measured without contacting the metal thin film based on the ratio of the consumed power.
The double-sided eddy current scheme has been applied in recent development of resistance meters which are intended for use in small sheet resistance monitors to control quality of semiconductors in their manufacturing process. For example, Japanese Laid-Open Patent Application No. 6-69310/1994 (Tokukaihei 6-69310; published on Mar. 11, 1994) discloses a wafer probing system whereby a resistance meter is disposed in the loader section and positioned parallel to the direction in which a transport robot moves so that the resistivity of the wafer can be measured using the resistance meter while the wafer is being transported. The laid-open patent application does not explicitly describe that the resistance meter is based on the double-sided eddy current scheme whereby the resistivity is measured without direct contact. It is inferred from the attached drawings, however, that the invention may be reduced to practice using either a contact-type resistance meter based on, for example, the four probe technique or one based on the double-sided eddy current scheme.
In this measuring system, there is provided an operation flow where either the robot is temporarily halted to measure resistivity or the wafer is inserted into, or transported through, the resistance meter, to measure the resistivity while the wafer is moving.
However, in this prior art system, the resistance meter is lacking in adequate sensitivity to be installed in an existing semiconductor manufacturing process and needs a transporter with one or more axes, for example, which makes it difficult to ensure a suitable installation space. The resistance meter is therefore difficult to install in an existing semiconductor manufacturing process.
Japanese Laid-Open Patent Application No. 5-21382/1993 (Tokukaihei 5-21382; published on Jan. 29, 1993) discloses a similar sheet resistance meter of an eddy current detection type and its usage whereby eddy currents are induced in a metal thin film deposited by sputtering, and lines of a magnetic force produced by the eddy currents are detected without contacting the metal thin film to calculate the sheet resistance.
This laid-open patent application discloses a system installed in the sputtering device that is capable of controlling the sheet resistance of a metal thin film deposited on a wafer or another type of substrate by sputtering. The system includes a load lock chamber interconnected with a gate valve of the sputtering device, a transporter which transports a substrate into the load lock chamber, and a resistance meter which measures the sheet resistance of the metal thin film on the substrate transported by the transporter.
However, in the laid-open patent application, the substrate becomes very hot after the thin film is deposited. The sheet resistance meter of an eddy current detection type is critically affected by the heat through resultant expansion of the coil, temperature dependence of the sheet resistance, etc., and gives inconsistent readings. In addition, the installment of the resistance meter inside the load lock chamber makes maintenance work difficult and inefficient. These problems presumably make it difficult to make use of the sheet resistance value obtained from a previously deposited metal thin film in subsequent deposition.
Conceived of to address these problems was the non-contact sheet resistance meter of a one-sided eddy current detection type producing a magnetic field which acts on a test sample, such as a conducting thin film, to induce eddy currents in it, measures variations in the magnetic field due to the eddy currents, and detects the material of the test sample, i.e., properties of the thin film, through measurement of the sheet resistance.
The operating principle is explained below. First, it is well-known that when a coil 71 to which an alternating current is supplied from an alternating current generator 73 is moved close to a coil 72 (see FIG. 25), a voltage develops across the coil 72 due to electromagnetic induction effects and causes an alternating current to flow in the circuit partly constituted by the coil 72, i.e., an ammeter 74 and a load resistance 75.
Similarly, as shown in FIG. 26, when the coil 71 to which an alternating current supplied is moved close to a metal thin film 76 as a conductivity test sample, instead of the coil 72, eddy currents 77 are induced in the metal thin film 76. The impedance of the coil 71 (corresponding to the resistance for a direct current) is in reverse proportion to the amplitudes of the eddy currents 77 which, in turn, are determined by the distance to the target metal thin film 76, the material and dimensions of the metal thin film 76, and other factors. Thus, the impedance of the coil 71 can be measured and evaluated.
The sheet resistance meter of a one-sided eddy current detection type is so adapted to measure the sheet resistance by detecting dissipation (loss) caused by eddy currents based on variations of the impedance and converting the loss to a sheet resistance value. Specifically, the sheet resistance meter detects the energy loss due to eddy currents based on a difference xcex94V=|V0xe2x88x92V1|, for example, where V0 is a peak voltage when the permanently activated sensor head is in no vicinity of any other object, that is, placed at infinity, and V1 is a voltage when the sensor head is moved to a predetermined distance of the target metal thin film.
A disadvantage of this type of resistance meter is that it needs to produce a strong magnetic force to sufficiently cause the magnetic flux to concentrate and thus make it practically possible to measure the sheet resistance of the metal thin film, because the resistance meter of a one-sided eddy current detection type produces a magnetic force only on side of the metal thin film. Therefore, attempts are made to increase the strength of the magnetic force produced by the coil 71 by, for example, using electric power with a drive frequency as high as a few hundred kilohertz or even higher and increasing in the number of turns in the coil 71.
There are nevertheless still other problems with the sheet resistance meter. Readings on the sheet resistance meter drift over a long period of time, because the coil 71 is made of copper with a temperature coefficient of resistance of 0.0039 (see Table 1) which imparts temperature properties that are far from being satisfactory to the sheet resistance meter. Further, a higher frequency of the electric power supply causes the sheet resistance meter to generate accordingly more heat and makes it even more difficult to produce a stable voltage output over a long period of time.
The present invention has an objective to present non-contact, high precision sheet resistance meters of an eddy current detection type which can measure resistivity without halting the facilities or transport robots and without changing the flow in the existing semiconductor manufacturing process, and also to present methods of manufacturing electronic components incorporating the sheet resistance meters.
A sheet resistance meter of the present invention, in order to achieve the objectives, includes:
a sensor head including a coil which produces a magnetic field to induce eddy currents in a thin film formed on a substrate, so that lines of a magnetic force exerted by the magnetic field extend on one side of the substrate;
a sheet resistance detecting section, having a resistor for use in voltage detection, for detecting a sheet resistance of the thin film according to a variation of the magnetic field caused by the eddy currents;
a capacitor for achieving resonance with the coil; and
a temperature controlling section which controls a temperature of the coil.
With the arrangement, the sensor head is positioned at a predetermined place above one of the sides of the substrate so that the magnetic field produced by the coil reaches that side of the substrate, but does not extend out of the other side, and the lines of a magnetic force exerted by the magnetic field produced by the coil cross the thin film. Therefore, eddy currents are induced in the thin film due to the lines of magnetic force. Further, the provision of the capacitor which achieves resonance with the coil enables the production of a strong magnetic field.
In this arrangement, the eddy currents dissipate as Joule heat, and the impedance of the coil varies according to the eddy current loss. Hence, the voltage across the resistor for use in voltage detection changes depending on the variation of the impedance. The sheet resistance detecting section detects the sheet resistance of the thin film based on the change in the voltage.
Further, in the arrangement, the detection signal produced by the sensor head according to a variation of the magnetic field caused by the eddy currents is transmitted to the sheet resistance detecting section via, for example, a cable. Since a capacitor is provided considering the cable""s stray capacitance C, the sheet resistance meter retains good sensitivity over a long time of period and provides stable performance.
Further, in the arrangement, a temperature controlling section is provided to control the temperature of the coil. Therefore, by the temperature controlling section controlling the temperature of the coil at a constant value, for example, temperature fluctuations cause only a restrained drift in voltage values detected by the coil. The sheet resistance meter thereby produces stable results in detection during operation, especially, during continuous operation.
A method of manufacturing an electronic component of the present invention, in order to achieve the objectives, includes the step of forming a thin film on a substrate, using a thin film forming device,
wherein:
the sheet resistance of the thin film is measured using the sheet resistance meter, and the step of forming a thin film is controlled based on the measurement.
Therefore, in the method, the sheet resistance of a thin film on a substrate can be always detected in a stable manner, using the sheet resistance meter. The thin film forming step can be controlled quickly once an abnormality occurs in the sheet resistance of a formed thin film. Yields are thus improved in the manufacture of electronic components with a gate Ta or other thin films.